ALSA: hda/tas2781: Fix a potential race condition that causes a NULL pointer in case no efi.get_variable exsits

A a potential race condition reported by one of my customers that leads to
a NULL pointer dereference, where the call to efi.get_variable should be
guarded with efi_rt_services_supported() to ensure that function exists.

Fixes: 4fe238513407 ("ALSA: hda/tas2781: Move and unified the calibrated-data getting function for SPI and I2C into the tas2781_hda lib")
Signed-off-by: Shenghao Ding <[e-mail]>
Signed-off-by: Takashi Iwai <[e-mail]>

sound/hda/codecs/side-codecs/tas2781_hda.c

···
 	efi_status_t status;
 	int i;
 
+	if (!efi_rt_services_supported(EFI_RT_SUPPORTED_GET_VARIABLE)) {
+		dev_err(p->dev, "%s: NO EFI FOUND!\n", __func__);
+		return -EINVAL;
+	}
+
 	if (hda->catlog_id < LENOVO)
 		efi_guid = tasdev_fct_efi_guid[hda->catlog_id];
 

sound/hda/codecs/side-codecs/tas2781_hda_i2c.c

···
 	unsigned int attr;
 	int ret, i, j, k;
 
+	if (!efi_rt_services_supported(EFI_RT_SUPPORTED_GET_VARIABLE)) {
+		dev_err(p->dev, "%s: NO EFI FOUND!\n", __func__);
+		return -EINVAL;
+	}
+
 	cd->cali_dat_sz_per_dev = TAS2563_CAL_DATA_SIZE * TASDEV_CALIB_N;
 
 	/* extra byte for each device is the device number */